Riot police patrolled the city, closing off roads and using containers to shield diplomatic missions.

The emergence of Tahir-ul-Qadri – who lived quietly in Canada until a month ago - has electrified the country's political scene, sparking frantic manoeuvring by a government fearful his demands could delay elections due in the spring.

He left the eastern city of Lahore on Sunday afternoon with about 10,000 supporters and is scheduled to address a growing crowd in Islamabad on Monday night.

Supporters gathered all day in the city centre, carrying blankets and food, and promising not to leave until their demands were met.

Khalid Qureshi, who had travelled about 50 miles to attend the rally, said he was prepared to stay for weeks if not months.

"We want democracy," he said. "The system is corrupt and only lets rich people have power."

The government of Asif Ali Zardari is due to complete its five-year-term in March. Successful elections would then mark the first time power has been transferred from one elected government to another.

Western donors have largely given up on Mr Zardari's administration but stress the importance of completing the transition to democracy after years of military rule.

Dr Qadri is demanding the country be handed over to a caretaker administration which would be charged with pushing through reforms ahead of elections.

The prospect troubles many in Pakistan, who fear such a proposal would lead to months of wrangling and a delay in polls.

Dr Qadri's critics suspect he may even be acting in concert with the country's powerful generals, who have seized power three times in the past and are accused repeatedly of destabilising civilian governments.

Javed Siddiq, Islamabad editor of the Nawa-e-Waqt newspaper, said his sudden rise, ability to attract crowds and big advertising budget made many wonder about his backers.

"They think he has come with an agenda of a foreign power or it is army inspired," he said.